Preliminary and published data from this group show for the first time that patients' tumors grown in a SCID mouse/xenograft model can be highly sensitive to being killed by Apo2L/ TRAIL, a recently identified death ligand of the TNF family for which there is considerable pre-clinical optimism. However, our preliminary observations also show that some tumors are resistant to Apo2L/TRAIL,implying that certain patients may not benefit from Apo2L/TRAILtherapy. The overall goal of the proposed research is to obtain a clear understanding of the degree to which Apo2L/TRAIL sensitivity vs. resistance naturally occurs in patient tumors and to identify both markers for sensitivity vs. resistance as well as strategies for overcoming resistance. Using our patient tumor model, we will test the hypothesis that targeting the two, complementary apoptotic signaling pathways (i.e. extrinsic and intrinsic) simultaneously with Apo2L/TRAIL in combination with chemotherapy will strengthen the apoptotic signal and facilitate enhanced killing of resistant malignant cells. Furthermore, in tumors displaying a natural sensitivity to Apo2L/TRAIL,this reagent could increase the therapeutic effects of chemotherapy, thereby enabling lower doses and reduced side effects. We expect that combination therapy will target a heterogeneous population of malignant cells with differential levels of sensitivity to single agents alone and may thereby target a broader population of tumor cells. The integrated aims of this proposal will: Aim 1) characterize a panel of freshly obtained patient pancreatic and colon tumors with regard to their sensitivity to Apo2UTRAIL Aim 2) analyze apoptotic signaling pathways in Apo2L/TRAIL sensitive vs. resistant tumors to identify markers that will enable selection of patients who will benefit by this treatment; Aim 3) analyze and compare apoptotic signaling pathways during treatment with Apo2L/TRAILalone, chemotherapy alone or combination therapy to identify mechanisms by which these agents interact to enhance tumor killing. Because of the extensive amount of experience and preliminary data we have acquired, our group is in a unique position to perform this analysis of patient tumors for factors that control sensitivity/resistance to Apo2L/TRAIL Moreover, this information will provide practical, relevant knowledge in terms of the clinical use of Apo2L/TRAIL.